Hydrogen Storage Material and Method for Producing the Same

ABSTRACT

A crystalline Al phase and a crystalline TiH 2  phase each having a maximum length of 200 nm or less are dispersed in an amorphous phase containing an Al—Mg alloy to obtain a hydrogen storage material capable of reversibly storing and releasing hydrogen.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2010-021905 filed on Feb. 3, 2010, ofwhich the contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydrogen storage material capable ofreversibly storing or releasing hydrogen and a method for producing thesame.

2. Description of the Related Art

Fuel-cell vehicles are equipped with a fuel cell for generating anelectric power utilizing an electrochemical reaction between hydrogenand oxygen. Thus, a motor of the fuel-cell vehicle is actuated by theelectric power from the fuel cell to generate a driving force forrotating tires.

The oxygen can be obtained from the air, and the hydrogen is generallysupplied from a hydrogen storage vessel. Therefore, the fuel-cellvehicle is further equipped with the hydrogen storage vessel.

As the hydrogen storage vessel has a higher hydrogen storage capacity,the fuel-cell vehicle can be driven over a longer distance. However,when the fuel-cell vehicle contains an excessively large gas storagevessel, the vehicle disadvantageously has an increased weight, resultingin a high load on the fuel cell. From this viewpoint, various methodshave been studied for increasing the hydrogen storage capacity of thehydrogen storage vessel while preventing the volume increase. In one ofthe methods, a hydrogen storage material is placed inside the vessel.For example, AlH₃, which can store hydrogen at a high ratio ofapproximately 10% by weight based on its own weight, is reported as aneffective hydrogen storage material in Japanese Laid-Open PatentPublication No. 2004-018980 (particularly paragraphs [0060] to [0062]).

As shown in FIG. 17, a crystalline AlH₃ 1 has a microstructurecontaining approximately square-shaped matrix phases 2 and a grainboundary phase 3 disposed therebetween. In this case, the matrix phases2 have a side length t1 of approximately 100 μm, and the grain boundaryphase 3 has a width w1 of several micrometers and occupies only aseveral volume percent of the structure. In an X-ray diffractionmeasurement of the crystalline AlH₃, a sharp peak of at least one of α,β, and γ phases is observed in the diffraction pattern.

It should be noted that the matrix phases 2 are composed of AlH₃ havinga crystal lattice containing Al and H, and the grain boundary phase 3 iscomposed of a solid solution of H in an amorphous Al.

In the crystalline AlH₃ 1, hydrogen is stored in accordance with thefollowing formula (1), while the stored hydrogen is released inaccordance with the formula (2). The formulae (1) and (2) representreactions in an arbitrary storage/release site, and do not mean that allsites of the crystalline AlH₃ 1 are oxidized and reduced.

Al+3/2H₂→AlH₃  (1)

AlH₃→Al+3/2H₂  (2)

The reaction of the formula (2) can be relatively readily induced, butthat of the formula (1) cannot be readily induced. As described inJapanese Laid-Open Patent Publication No. 2004-018980 (particularlyparagraphs [0060] to [0062]), the hydrogen gas storage can be repeatedonly when the AlH₃ is doped with Ti and NaH and then ball-milled under ahydrogen pressure of 100 atm.

In addition, as described in Sergei K. Konovalov and Boris M. Bulychev,Inorganic Chemistry, 1995, 34, 172-175 (particularly page 173, rightcolumn, lines 26-28 and FIG. 2), when the Al is hydrogenated by H₂ gascontact in a gas-phase process, the contact has to be carried out undera high pressure of more than 2.5 GPa (about 25000 atm) at a temperatureof 280° C. to 300° C. or under a further high pressure of 4 to 6 GPa ata temperature of 450° C. to 550° C.

As described above, the crystalline AlH₃ is notably disadvantageous inthat it cannot readily store the hydrogen.

SUMMARY OF THE INVENTION

A general object of the present invention is to provide a hydrogenstorage material capable of reversibly storing and releasing hydrogen.

A principal object of the present invention is to provide a hydrogenstorage material capable of readily storing and releasing hydrogen.

Another object of the present invention is to provide a hydrogen storagematerial having a high hydrogen storage capacity and a method forproducing the same.

A further object of the present invention is to provide a method forproducing the hydrogen storage material.

According to an aspect of the present invention, there is provided ahydrogen storage material capable of reversibly storing and releasinghydrogen, comprising

an amorphous phase containing an Al—Mg alloy and

a crystalline Al phase and a crystalline TiH₂ phase each having amaximum length of 200 nm or less and dispersed in the amorphous phase.

The hydrogen storage material having such a structure can exhibit a highhydrogen storage capacity even under a relatively mild condition. Inother words, the hydrogen storage material requires only a low energyfor the hydrogen storage. In fact, in the hydrogen storage material ofthe present invention, the hydrogen storage can be started at a pressureof approximately 10 MPa (100 atm) and a temperature of approximately 60°C. In addition, the material can release hydrogen under this condition.

This is presumably because the amorphous phase has a volume larger thanthose of the other phases (i.e., the amorphous phase is used as a motherphase). When hydrogen is stored in the above crystalline AlH₃ (see FIG.17), the hydrogen storage is started in the amorphous grain boundaryphase as described above. Similarly, in the hydrogen storage material ofthe present invention, when the hydrogen storage is preferentiallycaused in the amorphous phase, since the amorphous phase is the motherphase having a volume larger than those of the other phases, it ispresumed that the hydrogen storage material can exhibit a high hydrogenstorage capacity even under the relatively mild condition.

Furthermore, as compared with materials using only Al, in the hydrogenstorage material of the present invention, adsorption of the hydrogenmolecules, dissociation of the adsorbed hydrogen molecules to hydrogenatoms, and diffusion of the dissociated hydrogen atoms into theamorphous phase are accelerated due to the presence of Mg. Thisincreases the hydrogen storage capacity.

In addition, in the present invention, the TiH₂ acts to accelerate theadsorption of the hydrogen molecules to the hydrogen storage materialand the release of the hydrogen molecules from the hydrogen storagematerial. Thus, the hydrogen storage material can store and releasehydrogen even under the relatively mild condition.

When a metal particle having a maximum diameter of 500 nm or less isdispersed in the amorphous phase, the above effects can be furtherimproved, so that the hydrogen storage capacity can be increased under apredetermined condition. The reason is thought to be that the metalparticle has an activity for storing hydrogen.

In this case, the hydrogen storage material can store and releasehydrogen even under a pressure of approximately 10 MPa (100 atm) and aroom temperature (25° C.)

The metal particle may contain any component as long as it can show theabove activity. Preferred examples of the components include Ni, Fe, Pd,and combinations of two or more thereof.

According to another aspect of the present invention, there is provideda method for producing a hydrogen storage material comprising acrystalline Al phase and a crystalline TiH₂ phase each having a maximumlength of 200 nm or less and dispersed in an amorphous phase containingan Al—Mg alloy, comprising:

mixing AlH₃, MgH₂, and TiH₂ to prepare a mixed powder,

ball-milling the mixed powder in a hydrogen atmosphere for 60 to 600minutes while applying a force of 5 G to 30 G (in which G isgravitational acceleration) to prepare a milled product, and

dehydrogenating the milled product to obtain the hydrogen storagematerial.

In this constitution, in ball milling, a great force of 5 G to 30 G isapplied to the mixed powder of the AlH₃, MgH₂, and TiH₂. By applying theforce, the matrix structure of the AlH₃ and MgH₂ can be converted to theamorphous Al—Mg alloy phase, and the crystalline Al phase and thecrystalline TiH₂ phase can be each distributed as a dispersed phasehaving a maximum length of 200 nm or less in the amorphous phase of themilled product.

Thus, in this constitution, the hydrogen storage material produced byapplying the force to the mixed powder in ball milling can store a largeamount of hydrogen under a relatively mild condition.

The ratio of the AlH₃ to the total of the MgH₂ and the TiH₂ in the mixedpowder is not particularly limited. For example, the weight ratio of theAlH₃ to the total of the MgH₂ and the TiH₂ may be 55/45 to 95/5. Theweight ratio of the MgH₂ to the TiH₂ is preferably 1/9 to 9/1.

As described above, when the metal particle having a maximum diameter of500 nm or less is dispersed in the amorphous phase (the mother phase),the hydrogen storage capacity can be increased under a predeterminedcondition. In this case, the metal particle having a maximum diameter of500 nm or less may be further added to when mixing the AlH₃, MgH₂, andTiH₂. Of course, the AlH₃, MgH₂, TiH₂, and metal particle may be mixedin random order.

In this case, the metal particle preferably contains Ni, Fe, Pd, or twoor more thereof. The components are excellent in the effect ofincreasing the hydrogen storage capacity as described above.

In the case of adding the metal particle, the ratio of the AlH₃ to thetotal of the MgH₂, TiH₂, and metal particle in the mixed powder is notparticularly limited. For example, the weight ratio of the AlH₃ to thetotal of the MgH₂, TiH₂, and metal particle may be 55/45 to 95/5.

As described above, in the present invention, since the amorphous phasecontaining Mg is used as the mother phase and the crystalline TiH₂ phaseis dispersed in the mother phase, the hydrogen storage capacity can beincreased even under the relatively mild condition. Thus, the hydrogenstorage material of the present invention can exhibit a high hydrogenstorage capacity even under low temperature and low pressure. This ispresumably because the Mg acts to accelerate the incorporation(absorption) of the hydrogen, the TiH₂ acts to accelerate the adsorptionof hydrogen to the hydrogen storage material and the release of hydrogenfrom the hydrogen storage material, and the hydrogen storage ispreferentially caused in the amorphous phase (the mother phase) having avolume larger than those of the other phases.

Therefore, in a gas storage vessel containing the hydrogen storagematerial, it is unnecessary to form a heating device or a particularstructure for improving the pressure resistance. As a result, thestructure of the gas storage vessel can be simplified to reduce theequipment investment.

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings in which preferredembodiments of the present invention are shown by way of illustrativeexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transmission electron microscope (TEM) photograph of ahydrogen storage material according to a first embodiment of the presentinvention;

FIG. 2 is an electron beam diffraction image obtained by a selected-areaanalysis of a light gray portion shown in FIG. 1;

FIG. 3 is an electron beam diffraction image obtained by a selected-areaanalysis of a dark gray portion shown in FIG. 1;

FIG. 4 is an electron beam diffraction image obtained by a selected-areaanalysis of a black portion shown in FIG. 1;

FIG. 5 is a schematic explanatory view showing a microstructure of thehydrogen storage material shown in FIGS. 1 to 4;

FIG. 6 is a schematic explanatory view showing a microstructure of ahydrogen storage material according to a second embodiment of thepresent invention;

FIG. 7 is a TEM photograph of the hydrogen storage material shown inFIG. 6;

FIG. 8 is an electron beam diffraction image obtained by a selected-areaanalysis of a gray portion shown in FIG. 7;

FIG. 9 is an electron beam diffraction image obtained by a selected-areaanalysis of a black portion “a” shown in FIG. 7;

FIG. 10 is an electron beam diffraction image obtained by aselected-area analysis of a black portion “b” shown in FIG. 7;

FIG. 11 is an electron beam diffraction image obtained by aselected-area analysis of a black portion “c” shown in FIG. 7;

FIG. 12 is an X-ray diffraction pattern of a final product obtained inExample 1;

FIG. 13 is a graph showing results of a hydrogen storage/releasemeasurement (a PCT measurement) of the final product;

FIG. 14 is an X-ray diffraction pattern of a final product obtained inExample 2;

FIG. 15 is a graph showing results of a PCT measurement of the finalproduct at 25° C.;

FIG. 16 is a graph showing results of a PCT measurement of the finalproduct at 60° C.; and

FIG. 17 is a schematic explanatory view showing a microstructure of acrystalline AlH₃.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several preferred embodiments of the hydrogen storage material and theproduction method of the present invention will be described in detailbelow with reference to the accompanying drawings.

FIG. 1 is a transmission electron microscope (TEM) photograph of ahydrogen storage material according to a first embodiment of the presentinvention. As shown in FIG. 1, in the TEM analysis, most of the hydrogenstorage material is composed of a light gray portion, and a dark gray(near-black) portion and a black portion are dispersed therein. Thelight gray portion corresponds to a mother phase, and the dark grayportion and the black portion each correspond to a dispersed phase.

FIG. 2 is an electron beam diffraction image obtained by a selected-areaanalysis of the light gray portion. A halo pattern is shown in FIG. 2,so that the light gray portion is an amorphous phase. In addition, in anenergy dispersive X-ray spectroscopy (EDS) of the light gray portion,the presence of Al and Mg is observed in the light gray portion. As madeclear from the results, the light gray portion (i.e., the mother phase)is composed of an amorphous Al—Mg alloy phase.

As shown in FIG. 3, in a selected-area analysis of the dark grayportion, a clear spot pattern indicating the crystallinity is observed.In addition, in an EDS analysis, Al is observed in the portion. Thus,the dark gray portion is composed of a crystalline Al phase.

Similarly, as shown in FIG. 4, in a selected-area analysis of the blackportion, a clear spot pattern indicating the crystallinity is observed.In addition, in an EDS analysis, the presence of Ti and H is observed inthe black portion. Thus, the black portion is identified as acrystalline TiH₂ phase.

Therefore, the hydrogen storage material has a structure containing themother phase of the amorphous Al—Mg alloy phase and the dispersed phasesof the crystalline Al phase and the crystalline TiH₂ phase.

FIG. 5 is a schematic explanatory view showing a microstructure of ahydrogen storage material 10 having the light gray portion (the motherphase), the dark gray portion (the first dispersed phase) and the blackportion (the second dispersed phase) shown in the above electron beamdiffraction images. In FIG. 5, the referential numbers 12, 14, and 16represent the mother phase, the first dispersed phase, and the seconddispersed phase, respectively.

As described above, when hydrogen is stored in the crystalline AlH₃ 1(see FIG. 17), the hydrogen storage is started in the amorphous grainboundary phase 3. Also in the hydrogen storage material 10 of the firstembodiment, the hydrogen storage is considered to be started in theamorphous mother phase 12.

As is clear from FIGS. 1 and 5, in the hydrogen storage material 10 ofthe first embodiment, the amorphous mother phase 12 has a remarkablyhigh volume ratio. Therefore, the hydrogen storage material 10 has alarge hydrogen storage site, and thus has a significantly high hydrogenstorage capacity.

In the mother phase 12, the Al and Mg are randomly distributed.Therefore, an energy required for hydrogenating Al in this hydrogenstorage site can be lower than an energy required for hydrogenating Alinto a crystalline AlH₃ in a gas-phase process. Thus, an energy requiredfor storing hydrogen in the mother phase 12 can be lower than thatrequired for storing hydrogen in the crystalline AlH₃. As a result, thehydrogen storage material 10 can readily store hydrogen.

The mother phase 12 contains the Mg. The amorphous phase containing theAl—Mg alloy can more readily adsorb hydrogen molecules as compared withamorphous phases containing only Al. In addition, the amorphous Al—Mgalloy phase is more excellent in the dissociation of the hydrogenmolecules into hydrogen atoms and the diffusion of the dissociatedhydrogen atoms to the inside. Thus, the process from the hydrogenadsorption onto the mother phase 12 to the hydrogen incorporation(storage) is accelerated due to the presence of the Mg.

In addition, the TiH₂ in the second dispersed phase 16 acts toaccelerate the adsorption of the hydrogen molecules to the hydrogenstorage material 10 and the release of the hydrogen molecules from thehydrogen storage material 10.

For the above reasons, as compared with the crystalline AlH₃ 1 shown inFIG. 17, the hydrogen storage material 10 can store a larger amount ofhydrogen even under a relatively mild condition at a hydrogen pressureof approximately 10 MPa (100 atm) and a temperature of approximately 60°C. In addition, it is unnecessary to subject the hydrogen storagematerial 10 to a ball milling treatment for storing hydrogen.

The first dispersed phase 14 (the crystalline Al phase) and the seconddispersed phase 16 (the crystalline TiH₂ phase) each have a maximumlength of 200 nm or less. In other words, the hydrogen storage material10 does not contain a first dispersed phase 14 or a second dispersedphase 16 having a length of more than 200 nm, which is measured in atwo-dimensional plane.

The hydrogen storage material 10 may be produced as follows.

AlH₃ is synthesized first.

For example, AlH₃ may be obtained by dissolving AlCl₂ in a diethyl ethersolution of LiAlH₄ to carry out a reaction therebetween at ambienttemperature. LiCl generated in the reaction is removed by filtration,and the filtrate is exposed to reduced pressure using a vacuum pump orthe like at room temperature to evaporate diethyl ether. Then, theresidue is dried under reduced pressure at a temperature of 40° C. to80° C. to obtain a solid powder of AlH₃. At this point, the AlH₃ iscomposed of a crystalline AlH₃. Then, the AlH₃ powder is mixed with MgH₂powder and a TiH₂ powder to prepare a mixed powder. The MgH₂ powder andTiH₂ powder are easily commercially available.

The ratio between the MgH₂ and TiH₂ is not particularly limited, and theweight ratio of the MgH₂ to the TiH₂ may be 1/9 to 9/1. Also the ratiobetween the AlH₃ and the total of the MgH₂ and TiH₂ is not particularlylimited, and the weight ratio of the AlH₃ to the total of the MgH₂ andTiH₂ may be 55/45 to 95/5.

The mixed powder is ball-milled in a hydrogen gas atmosphere whileapplying a force of 5 G to 30 G (in which G is gravitationalacceleration). Specifically, the mixed powder is enclosed in a pottogether with a crushing ball in the hydrogen atmosphere such that theinternal hydrogen pressure of the pot is 0.1 to 2 MPa.

Then, the pot is fixed between a press shaft and a rotatable table,which is disposed rotatably on a disc-shaped base plate of a planetaryball milling apparatus, and the disc-shaped base plate and the rotatabletable are both rotated.

In the planetary ball milling apparatus, the pot is rotated orbitally byrotation of the disc-shaped base plate and rotated on its axis by therotatable table. Thus, the pot is rotated orbitally around a rotaryshaft connected to the disc-shaped base plate, and is rotated on itsaxis around the press shaft. The force is applied to the mixed powder inthe pot by the orbital motion and the axis motion. The inside of the potis kept under the hydrogen atmosphere during the ball milling to preventgeneration of an undesired compound such as magnesium alanate Mg(AlH₄)₂,whereby the amorphous phase containing the Al—Mg alloy can be obtained.

The force of 5 G to 30 G can be applied by controlling the rotationspeed of the disc-shaped base plate or the rotatable table, thetreatment time, etc. For example, when the pot has a diameter of 80 mm,a height of 100 mm, and an internal volume of 80 ml, and the disc-shapedbase plate has a diameter of about 300 mm, the rotation speed of thedisc-shaped base plate (the orbital motion) may be 50 to 500 rpm, therotation speed of the rotatable table (the axis motion) may be 30 to1000 rpm, and both of the orbital motion and the axis motion may becarried out for 60 to 600 minutes.

In this embodiment, a high energy is applied to the crystalline AlH₃,MgH₂, and TiH₂ in this manner. As a result, the matrix structure of thecrystalline AlH₃ and MgH₂ is converted to the amorphous Al—Mg alloyphase, and the crystalline Al phase and the crystalline TiH₂ phase eachhaving a maximum length of 200 nm or less are distributed as the firstdispersed phase 14 and the second dispersed phase 16 in the amorphousphase (the mother phase 12) of the obtained milled product.

When the force applied in the ball milling is less than 5 G (the millingtime is less than 60 minutes under the above condition), the abovedescribed microstructure cannot be satisfactorily formed. On the otherhand, when the force is more than 30 G (the milling time is more than600 minutes under the above condition), the amorphous phase is oftenconverted to the crystalline phase, so that the mother phase 12 maycontain a large amount of the crystalline phase needing a high energy inthe hydrogen storage.

Then, the milled product is subjected to a dehydrogenation treatment toform the hydrogen storage sites, whereby the hydrogen storage material10 shown in FIGS. 1 to 4 is obtained. In an X-ray diffractionmeasurement of the hydrogen storage material 10, peaks of the Al andTiH₂ are observed.

As shown in the schematic explanatory structure view of FIG. 6 and theTEM photograph of FIG. 7, a hydrogen storage material 20 according to asecond embodiment may contain, in addition to the mother phase 12, thefirst dispersed phase 14, and the second dispersed phase 16, metalparticles 18 dispersed in the mother phase 12. The hydrogen storagematerial 20 of the second embodiment will be described below.

The metal particle 18 can be dispersed in the mother phase 12 by addingthe metal particle 18 in the preparation of the mixed powder of theAlH₃, MgH₂, and TiH₂ and by performing the ball milling under the abovecondition.

FIG. 8 is an electron beam diffraction image obtained by a selected-areaanalysis of the mother phase 12 shown as a gray portion in FIG. 7. Ahalo pattern is shown in FIG. 8, so that the mother phase 12 is anamorphous phase also in this embodiment. In addition, in an energydispersive X-ray spectroscopy (EDS) of the light gray portion, thepresence of Al and Mg is observed in the portion.

On the other hand, as shown in FIGS. 9 to 11, in a selected-areaanalysis of each of black portions a, b, and c shown in FIG. 7, a clearspot pattern is observed. In addition, in an EDS analysis, the presenceof a metal added as the metal particle, Al, and TiH₂ is observed in theblack portions a, b, and c, respectively, as shown in FIGS. 9 to 11.

The metal particle 18 is not particularly limited, and preferablycontains Ni, Fe or Pd. The metals can significantly accelerate theadsorption of hydrogen molecules, the dissociation to hydrogen atoms,and the diffusion into the mother phase 12. The metals are particularlyexcellent in activity for dissociating the adsorbed hydrogen moleculesto the hydrogen atoms. Furthermore, the metals can advantageouslyaccelerate the formation of the amorphous Al—Mg alloy phase in the ballmilling of the mixed powder containing the AlH₃ and MgH₂.

Of course, two or more of Ni, Fe and Pd may be used together in themetal particle 18.

The metal particle 18 has a maximum diameter of 500 nm or less. When themaximum diameter is more than 500 nm, the activity on the abovedescribed adsorption, dissociation and diffusion may be deteriorated.

The maximum diameter of the metal particle 18 may be 1 nm or more,because it is difficult to prepare the metal particle 18 with anexcessively small diameter. It is particularly preferred that the metalparticle 18 has a maximum diameter of 1 to 100 nm from the viewpoints ofavailability and activity.

Example 1

13 g of AlCl₃ was added to and dissolved in 300 ml of a diethyl ethersolution containing 1 mol/l of LiAlH₄, and was reacted at the ambienttemperature until gas generation stopped. Then, LiCl precipitated in thesolution was removed by filtration, and the filtrate was exposed toreduced pressure for 1 hour using a vacuum pump to evaporate diethylether. The residue was dried under reduced pressure for 1 hour at eachtemperature of 40° C., 60° C. and 80° C., to obtain 2 g of a particulatesynthetic product. The steps were repeated to prepare 6 g of AlH₃particles in total.

0.7 g of the prepared AlH₃ particles were mixed with 0.1 g of MgH₂ and0.2 g of TiH₂ in an agate mortar to prepare a mixed powder. In the mixedpowder, the weight ratio of the AlH₃:the MgH₂:the TiH₂ was 7:1:2.

The mixed powder was enclosed together with a crushing ball in a pothaving an outer diameter of 80 mm, a height of 100 mm, and an internalvolume of 80 ml. In this step, the enclosure was carried out in ahydrogen atmosphere, and hydrogen was introduced to the pot such thatthe internal hydrogen pressure of the pot was 1.5 MPa.

The pot was sandwiched between a press shaft and a rotatable table on adisc-shaped base plate of a planetary ball milling apparatus(manufactured by Fritsch, Germany), and subjected to a ball millingtreatment. The disc-shaped base plate had a diameter of 300 mm, and therotation speed thereof was 350 rpm. The rotation speed of the rotatabletable (i.e. the speed of rotation of the pot on its axis) was 800 rpm,and the ball milling time was 300 minutes. A force of 16 G was appliedto the mixed powder under the condition.

The ball-milled powder was dehydrogenated to produce a final product.The final product was subjected to an X-ray diffraction measurementusing an X-ray diffractometer manufactured by Bruker. The X-raydiffraction pattern of the final product is shown in FIG. 12.

As shown in FIG. 12, only peaks of Al and TiH₂ were observed, and peaksof Mg, AlH₃, and MgH₂ were not observed. This means that a crystallineMg, a crystalline Al—Mg alloy, AlH₃, and MgH₂ were not contained in thefinal product.

A TEM photograph of the final product is shown in FIG. 1. Incidentally,the acceleration voltage was 200 kV.

As described above, the electron beam diffraction image obtained by theselected-area analysis of the light gray portion of FIG. 1 is shown inFIG. 2, and the electron beam diffraction images obtained by theselected-area analysis of the dark gray portion and the black portionare shown in FIGS. 3 and 4 respectively. It is clear from FIGS. 2 to 4that the light gray portion (the mother phase) was an amorphous phase,and the dark gray portion (the first dispersed phase) and the blackportion (the second dispersed phase) were crystalline phases.

In an EDS analysis, the presence of Al and Mg was observed in the lightgray portion (the mother phase), the presence of Al was observed in thedark gray portion (the first dispersed phase), and the presence of TiH₂was observed in the black portion (the second dispersed phase). It isclear from the results that the final product contained the crystallineAl phase (the first dispersed phase) and the crystalline TiH₂ phase (thesecond dispersed phase) in the amorphous Al—Mg alloy phase (the motherphase).

Furthermore, TEM photographs of various areas of the final product wereanalyzed. As a result, in the crystalline Al phases (the first dispersedphases) distributed as islands in the amorphous phase, the maximumlength measured in a two-dimensional plane fell generally within a rangeof 5 to 50 nm and at most 200 nm.

Meanwhile, in the crystalline TiH₂ phases (the second dispersed phases)distributed as islands in the amorphous phase, the maximum lengthmeasured in a two-dimensional plane was generally within a range of 20to 100 nm and at most 200 nm.

Then, 0.3 g of the final product was subjected to a hydrogenstorage/release measurement (a PCT measurement) under an appliedhydrogen pressure of vacuum to 10 MPa, a measurement temperature of 60°C., and a convergence time of 30 minutes. The results are shown in FIG.13. It is clear from FIG. 13 that the final product stored about 0.62%by weight of hydrogen at a relatively low pressure of 9 MPa.

The hydrogen was repeatedly stored even at a low pressure, the amount ofthe repeatedly stored hydrogen increased with the pressure increase, anda plateau was not formed. Therefore, it was presumed that the hydrogenstorage was caused not by AlH₃ formation but by a solid solution ofhydrogen in the amorphous phase (the mother phase).

Furthermore, as shown in FIG. 13, the final product stored hydrogen evenunder a hydrogen pressure of approximately 10 MPa (100 atm) and atemperature of approximately 60° C., and released hydrogen under thesame condition. It is clear from the results that the final product wasan excellent hydrogen storage material capable of reversibly storing andreleasing hydrogen.

Example 2

0.7 g of the AlH₃ particles prepared in Example 1 were weighed and mixedwith 0.1 g of MgH₂, 0.17 g of TiH₂, and 0.03 g of fine Fe particleshaving a diameter of 10 to 30 nm in an agate mortar to prepare a mixedpowder. In the mixed powder, the weight ratio of AlH₃/MgH₂/TiH₂/Fe was7/1/1.7/0.3.

Then, the mixed powder was ball-milled in the same manner as Example 1,and the milled powder was dehydrogenated, to produce a final product ofExample 2. The X-ray diffraction pattern of the final product is shownin FIG. 14. As a result, also in this example, only peaks of Al and TiH₂were observed, and peaks of Mg, Fe, AlH₃, and MgH₂ were not observed.This means that a crystalline Mg, a crystalline Fe, a crystalline Al—Mgalloy, AlH₃, and MgH₂ were not contained in the final product.

A TEM photograph of the final product is shown in FIG. 7. Incidentally,the acceleration voltage was 200 kV in the same manner as above.

As described above, the electron beam diffraction image obtained by theselected-area analysis of the gray portion of FIG. 7 is shown in FIG. 8,and the electron beam diffraction images obtained by the selected-areaanalysis of the black portions a, b, and c are shown in FIGS. 9 to 11,respectively. It is clear from FIGS. 8 to 11 that the gray portion (themother phase) was an amorphous phase, and the black portion a (the metalparticle), the black portion b (the first dispersed phase), and theblack portion c (the second dispersed phase) were crystalline phases.

In an EDS analysis, the presence of Al and Mg was observed in the grayportion (the mother phase), the presence of Fe was observed in the blackportion a (the metal particle), the presence of Al was observed in theblack portion b (the first dispersed phase), and the presence of TiH₂was observed in the black portion c (the second dispersed phase). It isclear from the results that the final product contained the fine Feparticle (the metal particle), the crystalline Al phase (the firstdispersed phase), and the crystalline TiH₂ phase (the second dispersedphase) in the amorphous Al—Mg alloy phase (the mother phase).

Furthermore, TEM photographs of various areas of the final product wereanalyzed. As a result, in the crystalline Al phases (the first dispersedphases) distributed as islands in the amorphous phase, the maximumlength of measured in a two-dimensional plane generally fell within arange of 5 to 50 nm and was at most 200 nm.

The crystalline TiH₂ phases (the second dispersed phases) were alsodistributed as islands in the amorphous phase. The maximum length of thecrystalline TiH₂ phases measured in a two-dimensional plane generallyfell within a range of 20 to 100 nm and was at most 200 nm.

In addition, the fine Fe particles in the final product hadapproximately the same diameters as those added in the mixing step. Thediameters were generally within a range of 10 to 30 nm.

Then, 0.3 g of the final product was subjected to a PCT measurementunder an applied hydrogen pressure of vacuum to 10 MPa, a measurementtemperature of the room temperature 25° C. or 60° C., and a convergencetime of 30 minutes. The results obtained at the room temperature areshown in FIG. 15, and the results obtained at 60° C. are shown in FIG.16.

It is clear from FIGS. 15 and 16 that the final product stored asignificantly large amount (about 0.62% or 0.7% by weight) of hydrogenunder a relatively low pressure of 9 MPa and a relatively lowtemperature of the room temperature or 60° C. As is clear from this, thehydrogen storage amount could be further increased by dispersing thefine Fe particles in the mother phase.

In Example 2, as well as in Example 1, the hydrogen was repeatedlystored at a low pressure, the amount of the repeatedly stored hydrogenincreased with the pressure increase, and a plateau was not formed.Therefore, it was presumed that the hydrogen storage was caused not byAlH₃ formation but by a solid solution of hydrogen in the amorphousphase (the mother phase).

Furthermore, as shown in FIGS. 15 and 16, the final product storedhydrogen even under a hydrogen pressure of approximately 10 MPa (100atm) and a temperature of the room temperature or approximately 60° C.,and released hydrogen under the same condition. It is clear from theresults that the final product was an excellent hydrogen storagematerial capable of reversibly storing and releasing hydrogen.

Although certain preferred embodiments of the present invention havebeen shown and described in detail, it should be understood that variouschanges and modifications may be made therein without departing from thescope of the appended claims.

1. A hydrogen storage material capable of reversibly storing andreleasing hydrogen, comprising an amorphous phase containing an Al—Mgalloy and a crystalline Al phase and a crystalline TiH₂ phase eachhaving a maximum length of 200 nm or less and dispersed in the amorphousphase.
 2. The hydrogen storage material according to claim 1, furthercomprising a metal particle having a maximum diameter of 500 nm or lessand dispersed in the amorphous phase.
 3. The hydrogen storage materialaccording to claim 2, wherein the metal particle contains Ni, Fe, Pd, ortwo or more thereof.
 4. The hydrogen storage material according to claim1, wherein in a hydrogen storage/release measurement under an appliedhydrogen pressure of vacuum to 10 MPa, a measurement temperature of 60°C., and a convergence time of 30 minutes, the hydrogen storage materialstores a greater amount of hydrogen as the applied hydrogen pressureincreases without formation of a plateau.
 5. A method for producing ahydrogen storage material comprising a crystalline Al phase and acrystalline TiH₂ phase each having a maximum length of 200 nm or lessand dispersed in an amorphous phase containing an Al—Mg alloy,comprising: mixing AlH₃, MgH₂, and TiH₂ to prepare a mixed powder:ball-milling the mixed powder in a hydrogen atmosphere for 60 to 600minutes while applying a force of 5 G to 30 G (in which G isgravitational acceleration) to prepare a milled product: anddehydrogenating the milled product to obtain the hydrogen storagematerial.
 6. The method according to claim 5, wherein in mixing AlH₃,MgH₂, and TiH₂, a weight ratio of the AlH₃ to a total of the MgH₂ andthe TiH₂ is 55/45 to 95/5, and a weight ratio of the MgH₂ to the TiH₂ is1/9 to 9/1.
 7. The method according to claim 5, wherein in mixing AlH₃,MgH₂, and TiH₂, a metal particle having a maximum diameter of 500 nm orless is further added.
 8. A method according to claim 7, wherein themetal particle contains Ni, Fe, Pd, or two or more thereof.
 9. Themethod according to claim 7, wherein a weight ratio of the AlH₃ to atotal of the MgH₂, the TiH₂, and the metal particle is 55/45 to 95/5.10. The method according to claim 5, wherein the hydrogen atmosphere hasa hydrogen pressure of 0.1 to 2 MPa.